The northern third of the state and some eastern counties were mapped by
subtracting depth to bedrock as shown on the map of thickness
of unconsolidated deposits. The thickness map was used as a mask to isolate
portions of the digital elevation map. The
isolated portions were then reduced by 0, 50, 100, or 200 feet as required. The
resulting layers were then recombined to create the map above. More reliable
data from published sources were overlain on top, superseding the estimated
bedrock elevation.

The overall result is surprisingly seamless. Small-scale features in the
digitally-estimated areas must be taken with caution, because small hills of
glacial origin and some valleys cut entirely into glacial deposits may show up.
Also, there may be artifacts at county lines and mapped boundaries of different
drift depths. However, large features of substantial relief are almost surely real. For
example, it's not clear from the unconsolidated-deposit map whether some areas
of thick deposits in Barron, Shawano, and Marinette counties are moraines or
extensions of nearby filled valley systems. Subtracting the drift thickness from
the digital elevation reveals features so low they are certainly real valleys.